In the medical field, implantable leads are used with a wide variety of medical devices. For example, implantable leads are commonly used to form part of implantable cardiac system that provides therapeutic stimulation to the heart by delivering pacing, cardioversion, defibrillation or resynchronization pulses. The pulses can be delivered to the heart via electrodes disposed on the leads, e.g., typically near distal ends of the leads. In that case, the leads may position the electrodes with respect to various cardiac locations so that the pacemaker can deliver pulses to the appropriate locations. Leads may be used for sensing purposes, or for both sensing and stimulation purposes. Implantable leads are also used in neurological devices, muscular stimulation therapy, gastric system stimulators and devices that sense chemical conditions in a patient's blood.
Occasionally, patients that have implantable leads may benefit from a magnet resonance image being taken of a particular area of his or her body. Magnetic resonance imaging (MRI) techniques can achieve a very effective image of the soft tissues of the heart and vascular system. MRI procedures can also image these features without delivering ionizing radiation to the body of the patient, and, as a result, MRI procedures may be reliably and safely repeated. However, MRI devices may operate at frequencies of 10 megahertz or higher, which may cause energy to be transferred to the lead. In particular, the high frequency fields induce a voltage in the lead, causing the potential of the lead to be higher than the surrounding tissue. In effect, the lead behaves as an antenna. Current may flow from the electrode into the tissue proximate to the electrode due to induced voltage.